Silicon-on-insulator (SOI) structures are used in microelectronic device applications where the electrical and electronic interactions between the active device region and the underlying semiconductor structure are strongly discouraged. In a typical SOI structure, the buried oxide layer separates the Si over-layer (i.e., the SOI or device layer) from the Si substrate.
In complementary metal oxide semiconductor (CMOS) devices built on SOI, for instance, performance characteristics are known to be greatly improved. Specifically CMOS devices built on SOI can exhibit less junction capacitance and leakage, greater resistance to ionizing radiation, immunity to latch-up, etc. However, forming SOI structures is no simple matter.
Even after decades of research and development only a few methods are proven to be commercially viable. In one, called BESOI (bond-and-etch-back SOI), two Si wafers are oxidized at the surface and the oxidized surfaces are bonded together and then one of the two bonded wafers is etched to provide a thin SOI device layer. In this prior art method and its variations, as the wafer surfaces are oxidized before bonding, the buried oxide can be made to have any desired thickness. However, impurities at the bonded interface and the difficulty in achieving a thin, uniform Si over-layer through the etch-back process are major drawbacks. The terms “Si over-layer” and “SOI layer” may be interchangeably used in this application.
In another well-known method, called SIMOX (separation by implantation of oxygen), a selected dose of oxygen ions is directly implanted into a Si wafer, and then the wafer is annealed in an oxygen ambient at a high temperature so that the implanted oxygen is converted into a continuous buried oxide layer. The thickness of the buried oxide layer in the SIMOX method is mostly dependent on the implanted oxygen dose and the thermal oxidation conditions. Moreover, in SIMOX, the Si over-layer is thinned to a desired thickness during the thermal oxidation, after which the surface oxide is stripped off.
When the peak concentration of the implanted oxygen is very low (on the order of about 1E22 atoms/cm3 or less), however, the buried oxide typically becomes broken and discontinuous, as the growing oxide precipitates tend to ball up to minimize the surface energy. Such an SOI structure is shown, for example, in FIG. 1. In FIG. 1, reference numeral 100 denotes the Si-containing substrate layer, reference numeral 102 denotes the buried oxide and reference numeral 104 denotes the Si-containing over-layer of a prior art SOI structure. As such, it is generally very difficult to form a buried oxide layer thinner than 100 nm using conventional SIMOX processing.
In MOSFET device applications, the Si over-layer and the buried oxide underneath need to be made thinner as the device dimensions shrink, in order to better control short-channel effects. This means that the buried oxide in up coming generations of MOSFET devices needs to be far thinner than what the conventional SIMOX technology is capable of.